1. Field of the Invention
The present invention relates to an internal combustion engine control device.
2. Description of the Related Art
Some internal combustion engines are provided with a turbocharger and a low-pressure EGR passage for drawing a portion of exhaust gas as low-pressure EGR gas from the exhaust passage downstream of the turbine of the turbocharger and downstream of a NOx adsorber catalyst (which is hereinafter referred to simply as “NOx catalyst”) into the intake passage upstream of the compressor of the turbocharger. Such internal combustion engines are also provided with a high-pressure EGR passage for recirculating a portion of exhaust gas as high-pressure EGR gas from the exhaust passage upstream of the turbine and recycling the high-pressure EGR gas into the intake passage downstream of the compressor. A technology for controlling the amount of low-pressure EGR gas flowing through the low-pressure EGR passage and the amount of high-pressure EGR gas flowing through the high-pressure EGR passage based on the required engine load during low-temperature combustion is described in JP-A-2005-076456.
In the structure in which the low-pressure EGR gas is drawn from the exhaust passage downstream of the NOx catalyst, when a rich spike, i.e. a process of adding fuel as a reducing agent to the exhaust gas upstream of the NOx catalyst to reduce NOx and SOx adsorbed by the NOx catalyst, is performed, the oxygen concentration in the low-pressure EGR gas that is recirculated to the intake passage suddenly changes and the combustion in the internal combustion engine becomes unstable. To reduce the fluctuation in torque of the internal combustion engine, the timing of main fuel injection from fuel injection valves for injecting fuel into the cylinders of the internal combustion engine may be advanced or a pilot injection carried out. However, as described above, if a rich spike is carried out during the process of reducing a fluctuation in torque of the internal combustion engine, the fluctuation in torque increases to an uncontrollable level and the combustion in the internal combustion engine becomes unstable. Also, if the fluctuation in torque of the internal combustion engine is reduced as described above, the internal combustion engine produces a larger amount of smoke and undesirable combustion noise.